Blog Post

Revised Air Conditioner Condensate Calculator Available on

Our online calculator allows you to calculate how much condensate can be captured from an air conditioning system.

Back in 2008 when I wrote a series of articles for Environmental Building News on water, one of those articles, Alternative Water Sources: Supply-Side Solutions for Green Buildings, examined various ways of harvesting water and included an in-depth look at collecting air conditioner condensate.

Here's an excerpt from that article on how that condensate is generated:

Cooling systems rely on evaporator coils through which refrigerant fluid changes from liquid to vapor, cooling the coils in the process. Air blowing past the coils cools off as it goes by, and moisture from the air condenses on the coils. Condensate drains carry away the water, usually into the sewer. Instead of wasting it, more and more buildings, especially in parts of the country with hot, humid summers, are capturing that condensate for reuse. The city of San Antonio, Texas, has actively pursued this practice. It makes so much sense there because of the large cooling load and high humidity. The downtown Rivercenter Mall in San Antonio collects about 250 gallons of condensate per day, which is used to replenish the cooling tower losses, and the San Antonio Public Library collects about 1,400 gallons on condensate per day, which is used for irrigation. A six-month payback was calculated on the condensate-recovery system at the Rivercenter Mall.

When we ran that EBN article, we also provided an online calculator to assist designers or building owners estimate how much air-conditioner condensate could be recovered. Eddie Wilcut and Elliot Fry, of the San Antonio Water System (SAWS), developed the spreadsheet, and then Kelly Lucas on our staff "webified" it.

Users brought to our attention some problems with the condensate calculator, however, so we took it off our site.


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I'm pleased to report that the BuildingGreen Condensate Calculator is back up, following some refinements by the SAWS team and Kelly's work to incorporate those changes into the online version.

Have a look, and take it for a test drive. Feedback will be very welcome; use the comments field.

Alex also writes the weekly blogs on Alex's Cool Product of the Week, which profiles an interesting new green building product each week, and Energy Solutions. You can sign up to receive notices of these blogs by e-mail--enter your e-mail address in the upper right corner of any blog page.

Alex is founder of BuildingGreen, LLC and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.

Published September 25, 2010

(2010, September 25). Revised Air Conditioner Condensate Calculator Available on Retrieved from

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March 14, 2022 - 7:04 pm

Nifty calculator, thanks. I'm trying to estimate condensate  rate produced by a split "ductless" AC unit which recirculates the air in the room and I do not what to use for Percentage of Outside Air.

Thanks in advance for your help,


December 8, 2021 - 3:17 pm

The input and output conditions are for the air entering and exiting the air conditioning system. An engineer will design the HVAC system to deliver air at a prescribed temperature and relative humidity level; those would be the output conditions. The input conditions depend on the outdoor air and will vary; the calculator assumes average atmospheric conditions.

September 3, 2020 - 1:35 pm

Could you please explain the difference between the input and output conditions? Also, how is "Percentage of Outside Air" determined? 

June 29, 2020 - 10:35 am

Hi Caleb, 

I can try to respond to your questions about the calculator, though I haven't used it in a long time and I'm not a mechanical engineer. As you see it's now available as a downloadable Excel calculator, rather than an online tool.

Relative to your question about its applicability to a 3-ton chiller system, I think it would be applicable as long as the condensate is produced centrally at the chillers. If each of the packaged fan coil units has its own condensate drain, then making use of the generated condensate would be difficult. If you want to reach out to me directly, try me at

Let us know if the calculator works for you and makes sense.

June 29, 2020 - 9:22 am

Hi! Alex is the best person to answer your question, and he has semi-retired from the company, but I'll pass along your comments.

June 28, 2020 - 2:02 pm

Hi I have some questions how can i contact you?

June 26, 2020 - 5:56 pm

I am trying to use this to estimate condensate produced for a residential building in new york city. The cooling system consists of 3 chillers that send chilled water to around 500 fan coil units in the apartments. There are also around 50 packaged ac units, some using chilled water and some dx. Would this tool be appropriate for this situation? I am familiar with the parameters used, but have some questions. Thanks

March 23, 2018 - 10:00 am

I'm new to all of this, so I apologize for what may be a newbie question.  The inside/outside temperature and humidity cells are easy to determine as well as the tonnage of the system, but how do you find the "percentage of outside air" for the calculator?

June 7, 2016 - 6:42 am

Hi,can you pls advise me about the dowloading link for the ac condensate drain calculation ,before it was found in  online now its not showing.

June 7, 2016 - 9:32 am

Yes, Pratheeksh. The file didn't come over when we switched platforms recently. We're working on it. Watch this space!

June 1, 2016 - 8:59 am

This link must have gotten left behind when we updated our website. I'll ask our IT folks to look into it. Thanks!

June 1, 2016 - 8:30 am

dear sir,

please note that, the link mentioned below is not working

July 13, 2013 - 1:42 pm

I tried your new RH calculator, but the GPH condensate appeared to go DOWN, not UP, when I increased the inside RH, with all else remaining the same.  And when I  decreased the inside RH it resulted in an INCREASE GPH condensate. I could understand this if one is supposed to fill in the DESIRED inside RH, but not the actual RH.  Or, perhaps I'm just looking at something wrong.

October 11, 2010 - 1:13 pm

This is more a question than a comment: Is this tool applicable for residential use? If it is, how would I determine the indoor relative humidity? If it is not, where can I find a similar tool for residential projects?

September 27, 2010 - 9:01 am

This is a very impressive tool. Those who find it useful may also want to check out the various ROI calculators available on the Resources page at While every project is different, and the metrics aren't perfect, they may be useful for estimating startup costs and payback for green projects. I hope this helps.

September 28, 2010 - 11:19 am

Appears to be a good tool. Where from I can download it? If the fresh air is treated by a centralized FAHU. Does the quantity gives condensated collected from FAHU? What about the condensate in FCUs?
What if enthalpy wheels are used for heat recovery?


October 3, 2010 - 4:00 am

The calculator isn't something you download--it runs online. See the link towards the end of the story.

It estimates water collected off the cooling coils, regardless of what type of air handling unit they're in. An enthalpy wheel would change that calculation.